The invention relates to a process for the preparation of an alkali-metal benzoate, more in particular sodium benzoate, besides benzyl alcohol by hydrolysis of a benzyl benzoate.
A special object of the invention is to provide a suitable method of processing a contaminated benzyl benzoate obtained in the oxidation of a monoalkyl-benzene compound with a gas containing molecular oxygen. The oxidation of toluene is effected on a technical scale, and the benzyl benzoate then formed is the unsubstituted benzyl benzoate, i.e., both benzene rings of the benzyl benzoate are unsubstituted. Because of this technical importance, the present invention will be explained primarily with reference to the toluene oxidation and the contaminated unsubstituted benzyl benzoate so obtained. However, it will be understood that the present invention can also be applied to contaminated substituted benzyl benzoates that, for instance, may form, in the oxidation of other alkyl benzene compounds, particularly those having 1 to 4 carbon atoms in the alkyl group, whose benzene ring may contain other non-interfering substituents, for instance, one or more halogen, nitro or sulphonic-acid substituents. Examples include ethyl benzene, p-chlorotoluene, p-nitrotoluene, and p-toluene sulphonic acid.
The oxidation of toluene, referred to above, may be conducted either in the liquid phase with, e.g., a cobalt and/or manganese salt that is soluble in the reaction medium as a catalyst, or in the gaseous phase with, e.g., a catalyst based on an oxide of vanadium or another transition metal according to art-recognized procedures as described in Stanford Research Institute (SRI) reports No. 7 (1965), 29; 7A (1968), 241; No. 7B (1976), 53.
Part of all of the benzoic acid, together with all of the lower boiling point products, can be distilled from the reaction mixture. This reaction mixture contains benzoic acid, benzyl benzoate and other products with a higher boiling point than benzoic acid which products are jointly referred to herein as tar residue as well as unconverted toluene and by-products with a boiling point lower than benzoic acid, such as benzyl alcohol and benzaldehyde. The materials distilled from the reaction mixture can be processed further to isolate pure benzoic acid, so that contaminated benzyl benzoate remains as a residue in the form of a tar that may also contain benzoic acid. This residue may be used as the starting material for the process according to the present invention and from the viewpoint of commercial practice is the preferred starting material.
Contaminated benzyl benzoate that may also be used as a starting material for the process according to the present invention can also be formed when the distillate described above is heated, optionally in the presence of an esterification/re-esterification catalyst, such as sulfuric acid, phosphoric acid, boron trifluoride or zinc acetate, in order to esterify or re-esterify any benzyl alcohol and/or light benzyl esters, such as benzyl formiate and benzyl acetate, into benzyl benzoate. Annoying contaminations are formed especially when the mixture still contains benzaldehyde during this ester forming reaction.
It is very difficult to recover useful products from such a contaminated benzyl benzoate. Benzyl benzoate may be separated from the tar residue by distillation, but a pure benzyl-benzoate product can hardly be obtained in this manner. The present applicant has found that the component fluorenone of the tar gives rise to special difficulties, for it appears that it is virtually impossible to separate fluorenone from benzyl benzoate. In addition to fluorenone, other unidentified compounds may play a part as well. For instance, while it is possible to hydrolyze the benzyl benzoate, if so desired after it has been distilled from the tar residue, into benzyl alcohol and sodium benzoate using, for instance, aqueous sodium hydroxide solution, the sodium benzoate is very difficult to recover in a pure form, as some impurities persistently combine with it. Sodium benzoate in a pure form is an important and valuable commercial product for which there is a substantial market. One of its uses is as an important preservative.